[Bug] 500 Internal Server Error on /openapi.json: PydanticSchemaGenerationError for ClientSession

[AITutor3]

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Desktop (please complete the following information):

• OS: [e.g. macOS, Linux, Windows]
• Python version(python -V):
• ADK version(pip show google-adk):

Model Information:

• Are you using LiteLLM: Yes/No
• Which model is being used(e.g. gemini-2.5-pro)

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[AITutor3]

GitHub Issue Draft
Describe the bug I am encountering a 500 Internal Server Error when trying to access the /openapi.json endpoint (via /docs) after running adk api_server. This issue appears to be identical to #3164 and #3173, where the server fails to generate the Pydantic schema for certain classes, particularly those related to MCP (e.g., mcp.client.session.ClientSession).

To Reproduce

Install google-adk

Run adk api_server

Open http://127.0.0.1:8000/docs in a browser.

Terminal shows the following error trace: pydantic.errors.PydanticSchemaGenerationError: Unable to generate pydantic-core schema for <class 'mcp.client.session.ClientSession'>.

Expected behavior The FastAPI Swagger UI documentation should load correctly without any internal server errors.

Desktop (please complete the following information):

OS: [예: macOS, Linux, Windows]

Python version(python -V): [예: 3.12.1]

ADK version(pip show google-adk): [예: 1.18.0]

Model Information:

Are you using LiteLLM: [Yes/No]

Which model is being used: [예: gemini-2.0-flash]